Crude oil, also called petroleum, is constituted from a complex mixture of many different molecules of different size and shape. The molecules may come from different molecular groups, such as alkanes, aromatics, waxes, and asphaltenes. The chemical composition of a crude oil plays an important role in determining the value of the crude, as well as the strategy for efficient production.
Crude oils are classified primarily according to their geographic origin, their API (American Petroleum Institute) gravity, and their sulfur content. The geographic origin is of interest due to transportation costs. Well-known locations are West Texas Intermediate, Brent (North Sea), and Oman. The API gravity is a measure of the oil quality. The API gravity has been defined to scale with the price of the oil. As lighter oils yield more gasoline, they are considered to be higher quality and therefore more expensive. Lighter oils have higher values of API gravity.
Crude oil is typically processed by distillation at atmospheric pressure. This is why crude oils are also characterized by their distillation yields. Distillation discriminates compounds based on the boiling points and not their chemical structure. It is thus a discrimination method based on molecular physics. With increasing temperature, the standard fractions are C1 to C4 compounds, naphtha, kerosene, gas oil, and atmospheric residue, which cannot be further distilled without the risk of cracking.
The chemical composition of oils and their fractions are generally analyzed by chromatographic and spectroscopic methods. Gas chromatography (GC) is frequently used for fractionation of viscous oils, and in many cases also high-performance liquid chromatography (HPLC) and super-critical fluid chromatography (SFC). The most common spectroscopic method of analysis is infrared spectroscopy (IR) as it can identify different chemical groups. Similar detail is provided by high-resolution NMR spectroscopy (ASTM Test Method D5292: Aromatic Carbon Contents of Hydrocarbon Oils by High-Resolution Nuclear Magnetic Resonance Spectroscopy) without the need of a reference standard and to a lesser degree by low-resolution NMR (ASTM Test Method D4808: Hydrogen Content of Light Distillates, Middle Distillates, Gas Oils and Residua by Low Resolution Nuclear Magnetic Resonance Spectroscopy). Another approved test method for compositional analysis of hydrocarbons is mass spectrometry (MS).
Saturates are non-polar normal alkenes, or n-paraffins, branched alkanes or iso-paraffins, and cyclo-alkanes or naphtenes. They are the largest single source of hydrocarbon or petroleum waxes with paraffin wax as the major constituent. Aromatics contain one or more ring structures similar to benzene with connections by aromatic double bonds. They are chemically and physically very different from paraffins and naphtenes. Resins are heavy liquids or sticky amorphous solids which are soluble in the petroleum fluid. Resins are understood to be molecular precursors of asphaltenes. Resins have polar head groups that surround asphaltenes and aliphatic tails that extend into the oil. As such, they stabilize asphaltene dispersions in oil. They can be converted to asphaltenes upon oxidation. Asphaltenes are defined as the petroleum fraction that is insoluble in light alkanes but soluble in toluene or dichloromethane. Their molecular structure is ill-defined but contains aromatic polycyclic clusters variably substituted with alkyl groups as well as hetero-atoms like N, S, O, and trace metals like Ni, V, Fe. Asphaltenes are dispersed as colloids in the oil and crack before boiling.
In addition to their carbon and hydrogen components, many oils contain trace amounts of other elements such as rare earth elements (e.g., Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, GD, Tb, Dy, Ho, Er, Tm, Yb, Lu), which include the lanthanides. Complexes with lanthanides find use in NMR spectroscopy as chemical shift reagents which spread the frequency range of the NMR signals from different chemical groups over a larger range.
High-resolution NMR spectra of crude oils have been reported by Hasan et al. See, e.g., Hasan, M. U., et al., “Structural characterization of Saudi Arabian heavy crude oil by NMR spectroscopy,” Fuel 62, pp. 518-523 ((1983); Hasan, M. U., et al., “Structural characterization of Saudi Arabian medium crude oil by NMR spectroscopy,” Fuel 64, pp. 839-842 (1985); Hasan, et al., “Structural characterization of Saudi Arabian extra light and light crudes by 1H and 13C NMR spectroscopy,” Fuel 68, p. 801 (1989); and Hasan, M. U., et al., “Structural characterization of Panero crude oil by FT-infrared and 1H NMR spectroscopy,” Fuel Sci. Tech. Int. 10, pp. 1407-1420 (1992). Hasan et al. divide the proton chemical shift range into four windows, namely, a window for aromatic protons Harom (6.0-9 ppm), and three windows for aliphatic protons Hsat (0.0-4.0 ppm), namely Hα (2.0-4.0 ppm), Hβ (1.0-2.0 ppm), and Hγ (0.5-1.0 ppm). A slightly different definition of windows has been reported for asphaltenes: aromatic protons Harom (6.5-9 ppm), Hα (2.0-4.0 ppm), Hβ (1.0-2.0 ppm), and Hγ (−1.0-1.0 ppm). See, e.g., Scotti, R. and Montanari, L., “Molecular structures and intermolecular interaction of asphaltenes by FT-IR, NMR, EPR,” Structures and Dynamics of Asphaltenes (O. C. Mullins and E. Y. Sheu), Plenum Press, New York (1998). The fractions of protons in these ranges vary in an interlinked fashion with the composition of the oil. The spectroscopic signals may be quantified in terms of relative intensities. For the protons in asphaltenes, Dickinson defines 1/n=IHα/(IHα+IHβ+IHγ). See, Dickinson, E. M., “Structural comparison of petroleum fraction using proton and 13C NMR spectroscopy,” Fuel 59 pp. 290-294 (1988).
When measured at a constant applied field, the positions of lines in a high-field high-resolution 1H NMR spectrum may vary by up to 0.3 ppm and more due to susceptibility effects from the oils and from different filling heights in the sample tubes. When measured with respect to internal TMS (the tetramethylsilane standard for measuring chemical shift), there seem to be no obvious shift variations within better than 0.01 ppm accuracy. The achievable chemical-shift resolution increases with decreasing viscosity such that low-viscosity oils give spectra with the highest resolutions. At elevated temperature, the spectral resolution improves. When changing the temperature from room temperature to 50° C., the change in resolution is rather small while at 80° C. it greatly improves. The four different chemical shift regions can clearly be identified and the chemical shifts of the most prominent peaks and the amplitude ratios of the largest peaks of six oil samples are summarized in Table 1 below.
TABLE 1Chemical shifts δ relative to internal TMS of the largest peaksOilδHγ [ppm]δHβ [ppm]δarom1 [ppm]δarom2 [ppm]AHβ/AHγAHarom/AHγIHarom/Itotal [%]110.871.246.977.391.80.0185.0130.871.256.826.981.40.0254.7200.871.276.847.010.70.0333.8210.871.276.857.012.40.0182.2240.871.286.857.032.70.0050.7250.881.246.816.932.30.0284.2
The most prominent signals in dead oil are from the β and γ protons. They are separated by 0.4 ppm and promise to be detectable by downhole NMR spectroscopy with an instrument providing medium resolution as low as 0.2 ppm. Each of these lines is composed from contributions of thousands of molecules but the dominant contributions are from aliphatic CH2 for the β resonances and from CH3 for the γ resonance. Differing compositions of oils give rise to variations of the peak positions and line-shapes. The chemical shift of the strong peak of the γ protons shows the least variation among all the oils. This identifies this peak as an internal chemical shift reference for down-hole NMR spectroscopy. The higher the chemical shift, the larger the variation of the strongest peaks from oil to oil. However, an assignment to chemical groups of individual molecules is not possible due to the large number of molecules present in crude oils (up to 50,000).